Aim and design
The aim of this study is the translation and validation process of a Polish translation of PPSv2 (PPSv2-Polish). This is a prospective observational study of cancer patients performed by the hospice care team at St Lazarus Hospice, Krakow, Poland. All participants were evaluated twice during two consecutive days for test-retest reliability by the same palliative team member (a trained and experienced palliative care nurse, psychologist, or physiotherapist), who cared for the patient. Additionally, on the first evaluation, two different care providers (each time by different types of professionals) independently evaluated the same patient to establish inter-rater reliability values. Each patient’s evaluation encompassed the PPSv2-Polish, which was compared with 3 additional performance scales to accomplish its construct validity.
Two hundred adult patients (aged ≥ 18 years) who were consecutively admitted to an in-patient hospice, Polish native speakers, clinically stable according to the attending physician, able to communicate and complete the questionnaires, and without cognitive impairment, were recruited and enrolled in the study. The sample size for this survey was based on general guidelines for conducting qualitative research .
Palliative Performance Scale version 2 (PPSv2) and the Polish version of the tool
The PPS provides a functional assessment of a patient’s ambulation, activity level, evidence of disease, self-care, food/fluids intake, and level of consciousness. The PPS has 11 categories, from 100% (full mobile and healthy) to 0% (dead) in increments of 10%. In 2006, PPS version 2 (PPSv2) was introduced after clarification of instructions for its use .
A modified combined translation technique  of PPSv2 into Polish was applied, which consisted of 1) independent forward translations by a physician, a psychologist and a Polish native speaker, 2) team discussion on identified differences between these 3 versions until agreement, 3) independent backward translations by a physician, a psychologist and a native English speaker, and 4) second team discussion on any differences between the original and back-translated versions until all agreed that the two versions were semantically identical. This method was consistent with the Victoria Hospice translation guidelines sent to the authors. The discussed back-translated version was then preliminarily tested on patients to obtain the final version of the PPSv2 (PPSv2-Polish – see Additional file 1). The implementation process encompassed education of the medical staff participating in this study during one training session, based on the Victoria Hospice Society instructions, giving the opportunity to get feedback from the team members.
Karnofsky Performance Score (KPS)
The KPS ranking is an 11-point scale and runs from 100% - perfect health, to 0% - dead. While first published in 1948  to evaluate the ability to survive chemotherapy for cancer, it has recently undergone several evaluation adjustments . The KPS provides great consistency of ratings by different oncology professionals . It may also serve as a life survival predictor .
Eastern Cooperative Oncology Group (ECOG) Performance Status (ECOG PS)
This scale, also called the World Health Organization (WHO) score, was published in its current form by ECOG in 1982  to assess a patient’s level of functioning in terms of the ability to care for himself, daily activity, and physical ability, in order to measure the impact of the disease/treatment on performance status. It has a good intra and interobserver agreement in cancer patients' performance status assessment . It consists of 6 categories, from 0 - fully active, to 5 - dead, and is simpler to use, may be clinically preferable in comparison to the KPS,  and is widely used in the literature.
Barthel Activities of Daily Living (ADL) Index
This "simple index of independence" was published in 1965 for measuring the improvement during rehabilitation of the chronically ill" . The original version was modified in 1988 to a 20 point scale that measures in increments of 1 point: from 0 - fully dependent, to 20 - fully independent . The final score can be multiplied by 5 to obtain a 100 point score, and it is proposed that scores of 0–20 indicate "total" dependency, 21–60 indicate “severe” dependency, 61–90 indicate “moderate” dependency, and 91–99 indicates “slight” dependency. It is already widely used as the measurement of daily living activities and has become a standard measure of physical disability in practice . Ten categories are assessed: feeding, grooming, bathing, dressing, toilet use, presence of fecal incontinence, presence of urinary incontinence, transfers (e.g., moving from wheelchair to bed), walking on an even surface (or propelling a wheelchair if unable to walk), and ascending and descending stairs.
We summarized the baseline demographics using descriptive statistics and medians with interquartile ranges (IQR) in non-normally distributed data. A Wilcoxon signed-rank test was used to compare test-retest of ordinal data in one sample in test-retest and inter-rater comparison. Non-parametric data within subgroups of patients were compared using a Mann-Whitney U test. The strength of the relationship between the test-retest variables, and between the tools scores and survival time were calculated with the Spearman’s rank correlation coefficient. The inter-rater reliability was estimated using Cohen’s kappa statistics. Data were analyzed using STATISTICA 13.0 (TIBCO Software Inc. 2017) data analysis software. A P-value of < 0.05 was considered as the level for statistical significance. As there are no absolute rules for the sample size needed to validate a questionnaire a fair size of 200 patients was chosen .